Side Inlets to Improve Water Quality

Ditches convey surface runoff water and subsurface tile drainage from artificially drained agricultural lands and are important to the agricultural economy of Minnesota and other Midwestern states. However, traditional methods of surface and subsurface drainage often result in degraded water quality. There has been increased interest in developing Best Management Practices (BMPs) for drainage ditches. Ideally, BMPs mitigate the negative impact of artificial drainage while limiting their negative consequences on crops and farm management. A potential BMP is the design or retrofitting of side inlets. Side inlets serve as surface runoff outlets for agricultural lands to drainage ditches. They are generally located adjacent to ditch berms in small depressions. They can be designed or retrofitted to temporarily store surface runoff, decreasing downstream peak flow rates and reducing the sediment contributions of croplands. There are many design variations of side inlet controls and include slotted standpipes, rock inlets, rock weirs and high-density drainage coils. The Hickenbottom riser was selected as a representative of a slotted pipe design. The effectiveness of alternative designs for side inlets was the primary focus of this study. The key tasks were to 1) develop a DEM-based method for the identifying the location of side inlets in the watershed and for characterizing their physical features, 2) to collect field data on the hydraulic characteristics and sedimentation processes for the different types of side inlets, 3) to use a simulation model for individual sites to rigorously investigate the impact of these inlets on the peak outflow rates and the effluent sediment loads, and 4) to assess the cumulative impact of multiple side inlets on flow rates and sediment yields at larger watershed scales. The types of side inlets considered in this study were the widely used existing straight pipes and alternative designs consisting of flush pipes, Hickenbottom risers, rock inlets, and rock weirs.